Read the last post, especially the second comment on the alleged "armored" aggressor in the Aurora shooting. As you all know, the aggressor was only wearing "tacticool" wardrobe items — no soft or plate armor. What I have said on the podcast and what we will say on TBD is that when faced with an aggressor who appears to be armored, TAKE THE CENTER MASS SHOTS IMMEDIATELY. If the aggressor is armored, the center mass shots will at worst buy you some time for a more precise shot and at best either spin the aggressor or knock him/her to the ground, giving you clearer shots at the unarmored areas — side, hips, head, legs.
Remember, you are probably not fighting a Navy SEAL or a Force Recon Marine who can take the pounding of rounds slamming into armor and keep going. Likely the aggressor has never been shot or even shot at. I would go as far as saying after examining active shooter situations it is likely the aggressor has never even taken a punch. I fall back on that great Zen philosopher Mike Tyson who noted that, "Everyone has a plan until they get hit in the face." Pain can be a massive distraction, and you are in a position to deliver the pain.
IMMEDIATELY transition to either a "failure drill" — the headshot — or some sort of mobility kill based on whatever targets are available. A mobility kill, a shot that knocks the aggressor to the ground and removes his/her ability to move freely, still does not solve the problem! That problem is only solved when the aggressor cannot continue firing and the threat ceases.
If the aggressor is only "dressed for success" in contractor casual, it will become readily apparent by noting the spreading red stain.